Cellular telephone systems are increasingly used for transmission of data from various sources to subscribers. The data are usually requested by a subscriber and are usually transmitted to the cellular system from appropriate sources over the Internet, then transmitted through the core network and a radio access network (RAN) of the cellular system to the base station, with which the subscriber unit is connected, and finally radio-transmitted from the base station to the subscriber unit. In conformance with the Internet Protocol (IP), all data are transmitted asynchronously as packets. Data arriving at the base station are accumulated in buffer storage, where a storage bin is designated to each actively connected subscriber unit. The present invention is concerned with the final lag of transmission, namely from a buffer storage bin, over the base station's radio transmitter, to the subscriber's receiver.
Cellular data transmission differs from cellular voice transmission in several aspects: (a) The required rate of transmission, in the case of data, varies greatly with time, as well as among subscriber units, while for voice it remains constant. (b) Moreover, the maximum required rate, for data, may exceed that for voice by orders of magnitude. (c) Unlike voice, data need not, generally, be transmitted continuously, but may be transmitted in bursts (i.e. many packets in close succession), with considerable intervals between them; however certain types of data applications (notably streaming types) have different tolerances to lengths of intervals. (d) Various subscribers may be given different degrees of quality of service (QoS)—for example, in terms of guaranteed minimum transmission rate. In order to accommodate these characteristics, new operating standards are being introduced to cellular systems. In particular, systems using the Code Division Multiple Access (CDMA) mode of transmission, have a new standard, known as CDMA 2000.
As is known, a channel in a CDMA system is defined by a particular code out of a set of N mutually orthogonal codes, known as a Walsh set. According to the CDMA 2000 standard there are defined, for any one radio transmission facility (e.g. a radio carrier), a set of N=2n fundamental channels, in terms of N Walsh codes, where N is typically 64 (n=6). Typically, a fundamental channel (FCH, also known as 1x channel, which is similar to a regular voice channel) carries data at a rate of about 10 Kbits per second. By defining suitable common subsets of the codes, fundamental channels are combinable, in a hierarchical manner into higher-rate (i.e. wider-bandwidth) channels as follows: A set of N/2 (e.g. 32) 2x channels, each carrying about 20 Kbits per second; a set of N/4 (e.g. 16) 4x channels, each carrying about 40 Kbits per second; and so on. The levels are also designated by a parameter k, whose value is 0 for the lowest level (i.e. for FCH), 1 for the second level, 2 for the third level and so on. The transmission rate (TR) at any level is then simply that of the lowest level (e.g. 10 Kb/s) multiplied by 2k. The corresponding Walsh codes are designed as a hierarchical binary structure, illustrated in FIG. 1, wherein, at each level, a code is a subset of certain two codes at the next lower level. Thus, for example, a 4x code is a subset of four related fundamental channel codes (two levels lower). The relationship between channels at the various levels, as determined by the code structure, is sometimes expressed in genealogical terms, such as “father”, “brother”, “son”, “grandson”, etc. When any particular wide channel is used, all related channels at the lower levels (i.e. all the “offsprings”), as well as all the directly related channels at the higher levels (i.e. all the “ancestors” in direct lineage) are not available; it is noted, though, that other related channels (such as “brothers” and “uncles” and their offsprings) are available, if not otherwise allocated. At any time, any available channel at any level may be allocable to any active subscriber unit, subject to the hierarchy discussed above and to certain constraints discussed below.
In common with other cellular systems, in a system operating under the CDMA 2000 standard, the signal power transmitted to any subscriber unit (SU) is a function of the transmission conditions (which depend, inter alia, on the distance between the subscriber unit and the base station), whereby the power is adjusted to maintain a given received signal-to-noise ratio. The signal-to-noise ratio is, however, also a function of the channel bandwidth and thus of the rate level of the channel; the higher the rate, and thus the wider the band, the higher the noise level and thus the higher the required transmission power. It is also noted that the total power of all signals transmitted at any instant is subject to a maximum value, characteristic of the transmitter.
Periodically channels must be allocated to subscriber units in order to carry to them data, addressed to them that have accumulated in the buffer storage. The duration of the period is in the order of a few hundred milliseconds. According to current practice, allocation is carried out, by a so-called scheduling engine, near the beginning of each period typically as follows: A storage bin is selected at random and the amount of data accumulated therein is compared with a series of threshold values; according to the outcome, a commensurate transmission rate, if any, is determined. If a channel of that rate is available, it is allocated to the subscriber unit corresponding to the bin, provided that the power required to transmit it to the subscriber unit does not cause the total power to exceed the maximum. Failing this, a channel of half the determined rate, if available, is allocated to that subscriber unit, again subject to the power test; and so on. Next, another bin is randomly chosen and the same process is followed. This cycle is repeated until there are no channels left or until any allocation would cause the maximum available power to be exceeded or until there is no data waiting in storage. Variations of this procedure are also in common practice.
Allocation procedures in present practice, as outlined above, do not optimally utilize the limited transmission resources, which are the overall data rate capacity and the maximum overall power. In particular, they allow spending an inappropriately large portion of the power on high rate transmission to subscriber units having poor radio reception; they also cause transmission rates to be dependent solely on buffered data sizes, which by themselves are random, and possibly on random selection. Moreover, procedures in present practice do not generally include QoS considerations and also cannot be geared to any business policy, such as would control transmission so as to maximize some variable (which may, for example, be overall transmission rate or overall revenue).